The present invention relates to a process of preparing cured latex-based compositions and, more particularly, relates to a process for in situ formation of cured latex gaskets in thermally deformable container closures.
It is common in the art of container closures to provide a gasket or "liner" about the periphery of the underside of the closure which functions to provide a tight seal between the closure and the upper rim of the closed container. The gasket is preferably of a resilient material which is capable of forming an air-tight seal with the container rim and which allows for resealing the closure after opening.
For the larger sized container closures e.g., those used for drums and 5 gallon pails, a latex gasket is often preferred, based on performance and cost factors. In general, latexes can be used to provide foamed gaskets having desired resiliency and inertness and, inasmuch as relatively large quantities of gasket material are required in such applications, latexes offer attractive economies as compared to other prospective gasket materials.
Gaskets have heretofore been incorporated into container closures either by insertion of a preformed gasket into the closure, generally involving insertion into a channel located about the periphery of the closure, or by introduction of a fluid latex into such a channel and in situ gelling, foaming and curing of the latex by application of sufficient heat. The former practice suffers from the disadvantage of requiring considerable expenditure for equipment and labor, both to preform the gasket and to insert same into the closure.
The latter practice, while generally more satisfactory when used with metal closures, has proven unsatisfactory for use with plastic closures. More specifically, temperatures above 200.degree. F. are required for curing the latex and at such temperatures plastic closures can undergo severe distortion rendering them completely unsuitable for their intended use. Since the use of plastic closures and containers has become widespread, it would be desirable to provide a means whereby latex gaskets could be formed in situ without causing the aforementioned distortion of the closure.
U.S. Pat. No. 4,304,744 to 1 relates to a method of forming a sealing gasket made from vinyl chloride resin plastisol in container closures made of a high-melting olefin polymer, especially polypropylene. As part of the method of the patent, the container closure is heated to a temperature from 5.degree. C. to 35.degree. C. below its melting point and, at this temperature, microwave heating is introduced to flux the plastisol. The conventional preheating of the closure is disclosed to permit complete fluxing of the plastisol by microwave heating as a result of minimization of heat loss from the plastisol during the microwave heating.